The metal thin film type magnetic recording tape has poor running durability as compared with that of the coating type magnetic recording tape. Various improvements have therefore been attempted. In particular, the surface shape of magnetic recording medium is a very important factor for increasing running durability. It is generally controlled by the surface shape of the support thereof or the undercoating layer formed on the support. The undercoating layer is formed by coating on a support a solution containing a mixture of fine particles, a binder, additives, a solvent and so on. Various materials and formation methods for the undercoating layer have been proposed, e.g., in (1) JP-A-59-30231 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), (2) JP-A-60-69816, (3) JP-A-61-168123, (4) JP-A-61-13427, and (5) JP-A-61-13428.
The above references (1), (2) and (3) relate to polymer particles used as non-magnetic particles of the undercoating layer, and references (4) and (5) aim at securing good contact between a magnetic recording tape and a magnetic head, by using a binder having a glass transition temperature lower than room temperature in the undercoating layer to inhibit curing of the magnetic recording tape.
Most of those proposals are concerned with the material of fine particles and the height and density of projections (i.e., protrusions) formed by fine particles, and the evaluation thereof is confined to the friction coefficient of a magnetic recording tape against a stainless guide pole and still durability. In actual running tests, however, head clogging, head-staining and jitter phenomena often occur because most of the fine particles hitherto proposed have insufficient effects on such problems. On the other hand, there were few reports on the effects of binders in the undercoating layer. Though binders having a Tg lower than room temperature were used in the foregoing references (4) and (5), their effect on running durability was not disclosed therein. Additionally, surface properties deteriorated when a binder having a Tg lower than room temperature was used in a magnetic recording medium having ordinary surface roughness. This is true because the surface of the undercoating layer suffers fine variation in shape under the influence of the back side of the support when the support is rolled up after coating of the undercoating layer.
Moreover, the ferromagnetic metal thin film type magnetic recording medium suffers from the defect that the ferromagnetic metal thin film thereof is easily cracked because of its tendency to have weak resistance against deformation, particularly instantaneous deformation. Therefore, an effective means to realize the compatibility of still durability with tape-running durability has been needed.